A predictable animal model of human cataract has been established in the deer mouse, Peromyscus maniculatus. A strain carrying an autosomal recessive trait for cataract has been developed. These animals have webbed toes, but are otherwise normal except for their eyes. By selective breeding, frequence and speed of cataract has been increased. Webbed-toed animals have a high frequency (88%) of abnormality of closure of the fetal fissure of the eye. This results in colobomas of the optic nerve, retina, choroid and ciliary body. Retinal dysplasia, myelinated nerve fibers in the retina, and persistence and hyperplasia of the vitreous vascular system occur. The lens has disruption of its normal alignment of cells in a lens bow; the lens epithelium migrates backward to the posterior pole of the lens. Abnormal development of lens cells and opacity of lens fibers ensues to produce cataract. Histologic studies of the lens opacity have been done by light microscopy and are contemplated by electron microscopy.